Field of the Invention
The present invention relates to an internal combustion engine control apparatus and more particularly to an internal combustion engine control apparatus provided with an exhaust gas recirculation amount estimation apparatus fort estimating an exhaust gas recirculation amount in an internal combustion engine.
Description of the Related Art
In order to appropriately control an internal combustion engine (referred to also as an engine, hereinafter), it is important to accurately calculate a cylinder flow rate as the amount of air taken into a cylinder of the engine and to perform fuel control and ignition timing control corresponding to the cylinder flow rate. With regard to fuel control, when feedback control can be performed in such a way that, primarily, a fuel, the amount of which causes a desired air-fuel ratio for a cylinder flow rate, is injected, almost excellent controllability can be obtained; however, ignition timing control needs to be performed at an ignition advance angle which causes an maximum output (referred to as an MBT (Minimum Spark Advance for Best Torque), hereinafter) in accordance with not only the engine rotation speed and the amount of air taken into a cylinder but also other factors such as the engine temperature, whether or not a knock has occurred, the fuel property, and the exhaust gas recirculation ratio (referred to as an EGR ratio, hereinafter), which is the ratio of the exhaust gas recirculation amount (referred to as an EGR (Exhaust Gas Recirculation) amount, hereinafter) to the cylinder flow rate.
Among the foregoing factors that provide effects to the MBT, for example, the engine temperature and whether or not a knock has occurred can be detected by an engine coolant temperature sensor and a knock sensor, respectively; with regard to the fuel property, it can be determined whether the fuel is regular gasoline or high-octane gasoline, based on whether or not a knock has occurred.
Meanwhile, with regard to the EGR ratio, there are two methods, i.e., a method (referred to also as an external EGR) in which an exhaust gas recirculation valve (referred to as an EGR valve, hereinafter) is provided in an exhaust gas recirculation path (referred to as an EGR path, hereinafter) that connects an exhaust pipe with an intake pipe and the EGR amount is controlled based on the opening degree of the EGR valve and a method (referred to also as an internal EGR) in which a variable valve timing mechanism (referred to as a VVT (Variable Valve Timing mechanism), hereinafter), which makes the opening timings of an intake valve and an exhaust valve variable, is provided and depending on the opening/closing timing of the VVT, an overlap period, during which the intake valve and the exhaust valve are concurrently opened, is changed so that the amount of EGR, which is caused by remaining exhaust gas in the cylinder, is controlled; in some cases, both the methods are concurrently utilized. In the external EGR control method, the EGR ratio can approximately be calculated from the opening degree of the EGR valve, the exhaust pressure, and the air-intake pipe pressure. In the following explanation, an EGR and an EGR ratio, when simply expressed in this manner, denote an external EGR and an external EGR ratio, respectively.
In recent years, it has become common that engine control is performed utilizing, as an index, the output torque of an engine; even when the output torque is estimated, the thermal efficiency changes in accordance with the cylinder flow rate and the EGR ratio. Accordingly, in order to calculate the foregoing MBT and furthermore in order to estimate the torque and the thermal efficiency, it is required to accurately calculate the cylinder flow rate and the EGR ratio. In order to accurately obtain the EGR ratio, it is required to accurately calculate the EGR flow rate.
Patent Document 1 discloses an EGR ratio estimation apparatus that calculates the EGR flow rate and estimates the EGR ratio, based on an exhaust gas amount obtained from the opening area of an EGR valve and an exhaust gas amount obtained from an opening area command value for the EGR valve. With a simple configuration, the apparatus disclosed in Patent Document 1 can calculate an EGR flow rate, by utilizing a preliminarily provided EGR-valve opening degree vs. flow rate characteristic and the opening area of an EGR valve.